1. Field of the Invention
This invention relates to a novel type of wide angle lens having excellent image forming performance suitable as a picture-taking lens for cameras of large size, say 4".times.5" or so.
2. Description of the Prior Art
In order to well correct distortion and curvature of image field throughout 100.degree. or more, it is usually necessary to adopt a lens system of the type in which lenses are symmetrically disposed with a stop intervening therebetween. It is well known that such a symmetrical type of lens was realized by L. Bertele long ago.
However, the conventional lens system of this type has a problem in that the entire lens system is rendered bulky by the concave lenses occupying the opposite ends of the lens system. When swing and tilt photography is to be carried out with such a lens system mounted, for example, on a large camera, the great diameter of the last lens causes the lens to touch the bellows of the camera, or if not, causes the marginal light to be widely diverted, thus rendering swing and tilt photography impossible. Of course, such a disadvantage is not encountered by a camera which permits the back-focus (hereinafter referred to as B.f.) to be great like the retrofocus, but in the symmetrical type wherein a concave lens is disposed most adjacent to the image plane, the B.f. is necessarily short, so that the above-noted disadvantage is encountered.
As an ultra-wide angle lens for large cameras, it is therefore desirable to reduce the size of the lens system and also to minimize the effective aperture of the last lens in particular (the effective aperture on the last tangential plane).
On the other hand, when viewed in terms of the aberrations to be corrected, it is often the case that the lens is used at small aperture (because the influence of the granularity of the film is small as a result of the great dimensions of the picture plane), and for the purpose of providing a representation and depth of the minute structure of the object to be photographed; but it is still necessary to correct distortion, the curvature of image field, etc., as far as possible. It is also desirable that the relative aperture be as great as possible to facilitate focusing.
Generally, however, the smaller the lens system, the worse these aberrations become.